Traditional methods of osteosynthesis and osteotomy used permanent metal implants made of steel or titanium. However, since these durable metal implants represent a foreign body, patients receiving them are potentially at a greater risk of a local inflammation. Moreover, while these implants tend to permanently protect healing bones against mechanical exposure, this stress shielding-effect actually forestalls the stabilization of the bone tissue that needs mechanical loads to obtain and maintain its rigidity. One solution to this problem requires a follow up surgery to remove the permanent metal implants. But such follow up surgeries increase the risk of re-fracture of the healing bones, and/or cause the patients to suffer unnecessary inconveniences, including delayed recovery and incurrence of additional expenses.
Alternative implants using metallic magnesium and certain magnesium alloys have been shown to be biodegradable and potentially suitable for medical applications. However, because of the electrochemical activity of magnesium, the corrosion rates of such implants are highly dependent on factors such as implant composition, type of environment or site of implantation, and the surface condition of the implant (treated or untreated). When exposed to air the surface of untreated magnesium implants reacts with oxygen, building up a layer of magnesium hydroxide on the surface, thereby slowing down further chemical reactions. In saline media, such as in the environment of the human organism, untreated magnesium implants initially corrode very rapidly, producing high amounts of hydrogen gas and magnesium hydroxide. Uncontrolled corrosion of magnesium implants can cause premature failure of loaded implants due to stress corrosion cracking and/or due to corrosion fatigue. Moreover, because of the initial high gas release subcutaneous gas cavities might form. Thus, a need exists for magnesium based implants with improved corrosion performance.
The initial high gas release and the formation of gas bubbles in vivo can potentially be avoided by application of a coating to the surface of the magnesium implants prior to implantation. The coating would retard the rate of corrosion of the metal implants, thereby stabilizing the rate of gas release due to corrosion of the implants. Several attempts to improve corrosion performance of magnesium have been reported, including coating by anodization in solutions of concentrated alkaline hydroxides, or in solutions of hydrofluoric acid or acid fluoride salts.
Anodization of magnesium using base solutions of concentrated alkaline hydroxides is generally provided through the supply of a DC current at a range of 50 volts to 150 volts. A coating is formed on the magnesium through the formation of sparks within the bath. The tracking of the sparks across the surface of the magnesium element slowly places the coating onto the magnesium. The use of sparks throughout the process leads to a relatively high current usage and to significant heat absorption by the bath itself. Therefore, cooling may be necessary to reduce the temperature of the bath during the anodization process.
Use of hydrofluoric acid or acid fluoride salts in anodization of magnesium results in the formation of a protective layer of magnesium fluoride on the magnesium surface. This protective layer is not soluble in water and thus prevents further reaction of the magnesium metal.
Other methods for anodization of magnesium or alloys of magnesium incorporate other species into the film as it is formed on the surface of the magnesium. Some anodization processes use silicates and others use various ceramic materials.
However, many of the reported magnesium coatings might be toxic. Therefore, a need exists for biocompatible coating compositions and coating processes will produce resorbable biomaterial onto the surface of magnesium implants that cannot completely prevent the degradation process, so the performance of the implants can be modulated by how the implant is coated and/or the corrosion characteristic of the base material used to coat the implants.